How to demonstrate Archimedes Law in Blender?
If this is for teaching purposes, you could, instead, illustrate the following thought experiment:


From a perfectly accurate spring scale, dangle an infinitely thin string. The scale shows zero.
Dip the string into a glass of water.The scale still shows zero.
From infinitely thin, weightless material, construct a 1cm cubed box attached to the end of the string, underwater. The scale still shows zero.
Lower the glass. the scale now shows 1 gram.


The upward force on the box of water, while it was submerged, must have been 1 gram.

This, IMO, has the advantage of showing why the law applies, rather than simply demonstrating that Blender can simulate it.